Measuring the angle of incidence for radio waves from a remote transmitter (DX).

Background
For radio connections on short-wave over short distances, about 100 to 3000 km, the radio waves can travel by making a hop with a reflection on the ionosphere. In addition, the signal can make several hops between the earth’s surface and the ionosphere to arrive at the receiver. For each hop, losses occur, which is higher for lower frequencies. The losses occur when the signal travels through the ionosphere and at the reflection points. For ground reflection, conductivity is of great importance and losses are said to be 5 times greater (7 dB) when reflecting on rock / sand than against seawater. In addition, the loss of reflection is considerably greater at high waves compared to a calm sea. A good read:
http://www.astrosurf.com/luxorion/qsl-hf-tutorial-nm7m.htm .

Existing forecasts for radio connections are based on the above propagation models. In reality, the radio waves sometimes have to travel in a completely different way. For example, every morning I have contact with New Zealand over long path at 5 MHz with 10 W. With the wave propagation described above, SNR would be in the order of -100 dB. A likely scenario is that the signal travels most of the path in some way with low loss between different layers in the ionosphere. The signal strength is highest about half an hour before sunset in New Zealand. The sun has then been up for 1.5 hours in Stockholm. Anecdotally, then, the angle of incidence of the signal is high. The goal of the project is to verify this. 

Implementation
To measure the angle of incidence, the method is to measure the phase difference of the plane incident wave front at two points. If the measuring points are vertically above each other, the angle of incidence α can be calculated. If the separation between the receiving antennas is a quarter of a wavelength, the phase difference becomes β = 90° x sinα. This applies to the direct wave only. There is also a component from the ground reflection. For vertical polarization, it has a phase rotation of 180 at the reflection point for perfect ground.